I would argue your solution is exactly backwards. The job of the compositor is to draw a window how it asks to be drawn. The window in question is extending th

I would argue your solution is exactly backwards. The job of the compositor is to draw a window how it asks to be drawn. The window in question is extending that choice to the user: "how do you want me to look." It then tells the compositor to draw it in that fashion. I would argue that this is the correct place for this choice. If you use the compositor to control transparency, what you have is the window asking to be drawn in a certain way and the compositor saying "Eh, you'd look way better at 50% opacity". If it's the user controlling this it's even worse. Now they have to be able to write a regular expression on certain window classes for each application they want to modify, and have far less granularity in how the window is modified. The compositor exists to faithfully draw windows and place them on the screen according to what is requested. Only in exceptional cases should it be asked to do weird things with certain windows.